1. Field of the Invention
This invention relates to a process for converting certain hydrocarbons to a high yield of para-xylene utilizing a boron-containing crystalline aluminosilicate zeolite catalyst.
2. Description of the Prior Art
The disproportionation of aromatic hydrocarbons in the presence of zeolite catalysts has been described by Grandio et al. in the Oil and Gas Journal, Vol. 69, No. 48 (1971).
U.S. Pat. Nos. 3,126,422; 3,413,374; 3,598,878; 3,598,879 and 3,607,961 show vapor-phase disproportionation of toluene over various catalysts.
Alkylation of aromatic hydrocarbons with an olefin in the presence of a crystalline metallic aluminosilicate having uniform pore openings of about 6 to 15 Angstrom units is described in U.S. Pat. No. 2,290,607. U.S. Pat. No. 3,251,897 describes alkylation of aromatic hydrocarbons in the presence of X- or Y-type crystalline aluminosilicate zeolites, specifically such type zeolites wherein the cation is rare earth and/or hydrogen. U.S. Pat. Nos. 3,751,504 and 3,751,506 describe vapor phase alkylation of aromatic hydrocarbons with olefins, e.g. benzene with ethylene, in the presence of a ZSM-5 type zeolite catalyst.
In these prior art processes, the xylene product produced has the equilibrium composition of approximately 24 percent para, 54 percent of meta and 22 percent of ortho.
The alkylation of toluene with methanol in the presence of a cation exchanged zeolite Y has been described by Yashima et al. In the Journal of Catalysis 16, 273-280 (1970). These workers reported selective production of para-xylene over the approximate temperature range of 200.degree. to 275.degree. C. with the maximum yield of para-xylene in the mixture of xylenes, i.e., about 50 percent of the xylene product mixture, being ovserved at 225.degree. C. Higher temperatures were reported to result in an increase in the yield of meta-xylene and a decrease in the production of para- and ortho-xylene.
While the above-noted prior art is considered of interest in connection with the subject matter of the present invention, the conversion process described herein, utilizing a catalyst comprising a composite of a crystalline alumino-silicate zeolite and boron oxide, which zeolite has a silica/alumina ratio of at least about 12 and a constraint index within the approximate range of 1 to 12, has not, insofar as is known been heretofore described.
Of the xylene isomers, i.e., ortho, meta and para-xylene, meta-xylene is the least desired product, with ortho and para-xylene being the more desired products. Para-xylene is of particular value being useful in the manufacture of terephthalic acid which is an intermediate in the manufacture of synthetic fibers, such as "Dacron". Mixtures of xylene isomers, either alone or in further admixture with ethylbenzene, generally containing a concentration of about 24 weight percent para-xylene in the equilibrium mixture, having previously been separated by expensive superfraction and multistage refrigeration steps. Such process, as will be realized, has involved high operation costs and has a limited yield.